Cryogenic fluids such as a liquid He, O.sub.2, N.sub.2 and H.sub.2 are in widespread use. As far as possible they are surrounded by an insulating means to ensure that there is little transfer of heat into the cryogenic fluid. Francis et al. in U.S. Pat. No. 3,114,469 describe the thermal insulation of a cryogenic fluid storage container or Dewar. Porta (sic.: della Porta) in U.S. Pat. No. 4,546,798 and Schippl in UK Patent Application GB 2 139 311 A describe thermally insulated fluid transport pipes. In all these cases the thermal insulation is provided by means of a vacuum. Furthermore, in order to prevent degradation of the vacuum and hence loss of thermal insulation, it is customary to use a getter material within the evacuated space. This getter material serves to continuously sorb gases which are slowly released from the walls surrounding the vacuum space or from other components such as the known "superinsulations" which may also be present within the vacuum space.
Many different getter materials are known and have been used to maintain a vacuum for the thermal insulation of cryogenic fluids. della Porta supra lists at least five different suitable getter material among which there is a family of Zr--V--Fe ternary alloys described in more detail in Boffito et al. U.S. Pat. No. 4,312,669. One of the alloys described by Boffito, namely the alloy having the composition by weight of 70% Zr--24.6% V--5.4% Fe has found particular acceptance for the use as a getter material to maintain the vacuum in the insulation of cryogenic fluids. This getter material is available in commerce under the tradename of St 707 from SAES GETTERS S.p.A., Milan, Italy. Its widespread acceptance is probably due to the fact that it can be activated at relatively low temperatures of 500.degree. C. or less and still possesses a high gas pumping speed.
Unfortunately the use of such traditional getter materials has several disadvantages in the vacuum insulation of liquid hydrogen. Leaks may develop in the wall separating the vacuum insulation from the liquid hydrogen. In this case the liquid hydrogen evaporates and forms a relatively high pressure of hydrogen within the vacuum enclosure. This gaseous hydrogen reacts rapidly with the getter material due to its high gas pumping speed. As a consequence of this rapid chemical reaction the temperature of the getter material rapidly increases and may reach many hundreds of degrees centragrades. Simultaneously a leak may also occur in the wall which separates the vacuum insulation from the external atmosphere. This leak may be caused by the rapid increase in temperature of the getter itself or may occur for other reasons. Whatever the cause of an air leak into the vacuum enclosure there is formed an H.sub.2 --O.sub.2 mixture. This H.sub.2 --O.sub.2 mixture can react explosively on contact with the hot getter device with catastrophic consequences.